Piston for internal combustion engine, and internal combustion engine using the piston

ABSTRACT

A piston top surface ( 23 ) has a depression. A center region ( 28 A) is formed at the center of the depression to secure the diameter of a center tumble flow to be large. A side region ( 28 B) is formed on both sides of the piston top surface to generate a side tumble flow that is not influenced by a cylinder bore inner wall and has a flow line generally parallel to that of the center tumble flow.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piston for an internal combustionengine for an automobile or the like, and also to an internal combustionengine to which the piston is applied. More particularly, the presentinvention relates to an improvement in the shape of a depression formedin the top surface of a piston to maintain a tumble flow of intake airinside a cylinder.

2. Description of the Related Art

In conventional internal combustion engines (hereinafter occasionallyreferred simply to as “engines”) in which an air-fuel mixture iscombusted in a combustion chamber to generate power, a tumble flow(vertical vortex flow) generated in a cylinder is effectively utilizedto increase the combustion efficiency of fuel, and thus improves theoutput, the exhaust emission, the fuel consumption rate, and so forth.That is, the tumble flow agitates inside the cylinder to promoteevaporation and atomization of fuel, achieving an excellent fuelproperty of fuel in the combustion chamber.

Various configurations have been proposed so far to positively generatesuch a tumble flow. For example, Japanese Patent ApplicationPublications Nos. 9-105330 and 11-218026 (JP-A-9-105330 andJP-A-11-218026) disclose forming an intake port as a tumble port. Thatis, the flow line of intake air flowing into a cylinder is set closer tothe vertical direction by setting the axis of the intake port closer tothe vertical direction at a portion where it opens into the cylinder,allowing to obtain a large tumble flow.

Japanese Patent Application Publication No. 7-119472 (JP-A-7-119472) and

WO 00/77361 disclose providing a tumble control valve in an intakepassage, which opens and closes to generate a large tumble flow in acylinder as necessary. Specifically, the intake passage is partitionedby a separation wall (partition plate) into two, upper and lower flowpaths, and a tumble control valve for opening and closing the lower flowpath is provided. When it is necessary to generate a large tumble flowin the cylinder (for example when the engine is cold), the tumblecontrol valve is closed and intake air is sent from only the upper flowpath, so that the flow line of intake air flowing into the cylinder isset to a direction closer to the vertical direction, allowing to obtaina large tumble flow.

The engines in which means for positively generating a tumble flow suchas described above is implemented adopt a piston formed with adepression for maintaining the tumble flow in its top surface (pistoncrown). For example, JP-A-9-105330 discloses forming a depression with alarge length (the dimension in the direction along the tumble flow axis(the center line of a vortex flow)) in a region facing an exhaust valve,and forming a depression with a small length in a region facing anintake valve. JP-A-11-218026 discloses forming a circular bowl-shapeddepression in the piston top surface. JP-A-7-119472 discloses forming adepression of which outer peripheral shape in the direction along thetumble flow axis is generally arcuate, and of which outer peripheralshape in the direction perpendicular to the tumble flow axis isstraight, as viewed in plan. WO 00/77361 discloses forming in a pistontop surface a depression which is generally rectangular or generallytrapezoidal as viewed in plan. By forming such depressions in the pistontop surface, a tumble flow is guided by the piston top surface tomaintain the tumble flow.

The inventors of the present invention have found that the conventionaldepression shapes cannot achieve an optimum tumble flow, and thus havestudied on the shape of a depression to be formed in the piston topsurface. They also have considered the shape of a piston top surfacemore suitable to generate an effective tumble flow. A detaileddescription is as follows.

In the case where a depression is formed in a piston top surface tomaintain a tumble flow, the tumble flow flows along the shape of thedepression (the shape of the surface of a curved depression). Therefore,the outside diameter of the tumble flow is determined generallyaccording to the width of the depression (the dimension in the directionperpendicular to the tumble flow axis (the center line of a vortexflow): the dimension in the direction in which an intake valve and anexhaust valve face each other).

A part of the tumble flow generated in the central region in the axialdirection of the tumble flow (a part of the tumble flow generated at thecenter of the piston top surface, which is hereinafter referred to as“center tumble flow”) flows in a region where the depression width islarge, and therefore is hardly influenced by a cylinder bore inner wallbut flows along the shape of the depression. Therefore, a tumble flowwith a relatively large outside diameter is generated in the centralregion in the axial direction of the tumble flow by setting thedepression width larger. For example, in JP-A-11-218026 andJP-A-7-119472, a large tumble flow is generated in the central region inthe axial direction of the tumble flow by forming the outer peripheralshape of the depression as viewed in plan with curved lines.

On the other hand, a part of the tumble flow generated in both outerareas in the axial direction (on both sides of the axis) of the tumbleflow (a tumble flow generated at both ends of the piston top surface,which is hereinafter referred to as “side tumble flow”) is significantlyinfluenced by the cylinder bore inner wall existing adjacently in theaxial direction of the tumble flow. The side tumble flow influenced bythe cylinder bore inner wall includes a flow in the directionperpendicular to the tumble flow axis (a flow generally parallel to theflow line of the center tumble flow) and a flow toward the center sideof the piston top surface due to the influence of the cylinder boreinner wall.

FIG. 12 is a plan view of a piston top surface having a depression “a”of a typical shape, in which the directions of the center tumble flowand the side tumble flow flowing in the vicinity of the surface of thedepression are indicated by the arrows. In the drawing, “SE” representsa center tumble flow. “SA1” and “SA2” represent a side tumble flow, with“SA1” indicating a flow in the direction perpendicular to the tumbleflow axis and “SA2” indicating a flow toward the center side of thepiston top surface due to the influence of the cylinder bore inner wall.Also in FIG. 12, “b” indicates the position of an intake valve, while“c” indicates the position of an exhaust valve, respectively.

The energy given to the air (or air-fuel mixture) flowing into thecylinder from the intake port is evenly determined according to thecylinder bore diameter, the piston moving speed, and so forth. That is,the fluid energy of the entire tumble flow is even. Therefore, aneffective tumble flow is generated by utilizing the entirety of thegiven energy to create a flow in the direction perpendicular to thetumble flow axis without a loss.

However, some of the given energy is consumed to create a flow towardthe center side of the piston top surface, with the tumble flow actuallygenerated, in particular the side tumble flow, influenced by thecylinder bore inner wall as described above (see the side tumble flowSA2 in FIG. 12). That is, a part of the energy is consumed to create aflow other than in the direction perpendicular to the axis of the tumbleflow.

Thus, in the case where such a flow toward the center side of the pistontop surface is created, a sufficient flow in the direction perpendicularto the axis of the tumble flow cannot be obtained. In addition, thetumble flow toward the center side of the piston top surface and thecenter tumble flow interfere with each other, which wastefully consumesthe fluid energy of the center tumble flow.

In order to prevent generation of such a flow toward the center side ofthe piston top surface, other configurations are conceivable in which adepression in a piston top surface is not formed in the vicinity of thecylinder bore inner wall. An example of such configurations is shown inFIG. 13 (a plan view of a piston top surface).

With such a configuration, however, agitation cannot be performed in thecylinder in both outer areas in the axial direction of the tumble flow(regions “d” hatched in FIG. 13), making it difficult to promotesufficient evaporation and atomization of fuel.

SUMMARY OF THE INVENTION

The inventors of the present invention have found that the phenomenondescribed above (that the energy of a side tumble flow is consumed tocreate a flow toward the center side of a piston top surface) actuallyoccurs in a cylinder, and have focused on the possibility that thecombustion efficiency by a tumble flow can be significantly increased byrestricting generation of a tumble flow toward the center side of thepiston top surface. The inventors of the present invention also haveconsidered the possibility of sufficiently agitating intake air in bothouter areas in the axial direction of the tumble flow to sufficientlypromote evaporation and atomization of fuel in the entire cylinder.

The present invention provides a piston with a depression formaintaining a tumble flow formed in its top surface, in which thedepression has such a shape that can utilize most of the fluid energygiven to intake air to generate a tumble flow which flows in thedirection perpendicular to the tumble flow axis, and that can promoteevaporation and atomization of fuel generally in the entire cylinder.The present invention also provides an internal combustion engine usingthe piston.

That is, the depression for maintaining a tumble flow formed in thepiston top surface includes a part which functions to secure thediameter of a tumble flow (center tumble flow) to be large at the centerof the piston top surface, and a part which functions to generate atumble flow (side tumble flow) that is not influenced by the cylinderbore inner wall and that is generally parallel to the direction of theflow line of the center tumble flow on both sides of the piston topsurface.

A first aspect of the present invention provides a piston for aninternal combustion engine including a top surface having a depression.The depression includes a center region formed at a center in adirection along a tumble flow axis and a side region continuously formedon both outer sides of the center region in the direction along thetumble flow axis. The center region has a reduction part in which adimension of the center region in a width direction perpendicular to thetumble flow axis is gradually reduced toward a piston outer periphery inthe direction along the tumble flow axis. The side region has a constantpart extending generally in parallel to the tumble flow axis.

According to this configuration, intake air flowing into the cylindergenerates a tumble flow including a center tumble flow flowing along the“center region” and a side tumble flow flowing along the “side region.”

At the center of the “center region” in the direction along the tumbleflow axis, the depression width is secured to be relatively large, sothat a center tumble flow having a large diameter and a flow lineextending in the direction perpendicular to the tumble flow axis isgenerated. In addition, in the “reduction part” of the “center region,”the depression width is gradually reduced toward the piston outerperiphery in the direction along the tumble flow axis, so that thediameter of the tumble flow becomes gradually smaller toward the pistonouter periphery. That is, the diameter of the tumble flow becomesgradually smaller as the in-cylinder length (the length of the spaceinside the cylinder) in the direction perpendicular to the tumble flowaxis becomes gradually smaller toward the piston outer periphery, so asto generate an ideal center tumble flow (that is not influenced by thecylinder bore inner wall) with its flow line extending in the directionperpendicular to the tumble flow axis and not disturbed. The “reductionpart” also contributes to increasing the compression ratio in thecombustion chamber.

On the other hand, the “side region” has a “constant part” extendinggenerally in parallel to the tumble flow axis. The diameter of the sidetumble flow which flows in the “side region” does not become smallertoward the piston outer periphery, but is generally uniform over theentire “side region.” That is, a side tumble flow with a generallyuniform outside diameter smaller than that of the center tumble flow isgenerated in the “side region.” Thus, a side tumble flow hardlyinfluenced by the cylinder bore inner wall adjacent in the direction ofthe tumble flow axis is generated in the vicinity of the outer peripheryof the piston top surface, generating a tumble flow having a flow lineextending in the direction perpendicular to the tumble flow axis. As aresult, it is possible to promote evaporation and atomization of fuel inthe entire cylinder.

As described above, according to the first aspect of the presentinvention, it is possible to utilize most of the fluid energy given tointake air to generate a tumble flow which flows in the directionperpendicular to the tumble flow axis, and to promote evaporation andatomization of fuel in the entire cylinder.

A second aspect of the present invention provides a piston for aninternal combustion engine including: a top surface having a depressionfor maintaining a tumble flow; and an outer peripheral part having apiston pin hole through which a piston pin is to be inserted. Thedepression includes a center region formed at a center in a directionalong an axis of the piston pin hole and a side region continuouslyformed on both outer sides of the center region in the direction alongthe axis of the piston pin hole. The center region has a reduction partin which a dimension of the center region in a width directionperpendicular to the axis of the piston pin hole is gradually reducedtoward a piston outer periphery in the direction along the axis of thepiston pin hole. The side region has a constant part extending generallyin parallel to the axis of the piston pin hole.

Also according to this configuration, the same effect as that of thefirst aspect can be obtained. That is, it is possible to utilize most ofthe fluid energy given to intake air to generate a tumble flow whichflows in the direction perpendicular to the tumble flow axis (whichextends in parallel to the axis of the piston pin hole), and to promoteevaporation and atomization of fuel in the entire cylinder.

Further, a third aspect of the present invention provides a piston foran internal combustion engine including a top surface having adepression. An edge line defining an outer periphery of the depressionincludes a changing edge line along which a dimension of the depressionin a width direction perpendicular to a tumble flow axis becomes smallerfrom a center in a direction along the tumble flow axis toward a pistonouter periphery in the direction along the tumble flow axis, and aconstant edge line along which the dimension of the depression in thewidth direction is maintained to be generally constant toward the pistonouter periphery in the direction along the tumble flow axis. Thechanging edge line and the constant edge line are continuously formedwith an inflection part provided therebetween.

The piston may further include an outer peripheral part having a pistonpin hole through which a piston pin is to be inserted, and an edge linedefining an outer periphery of the depression includes a changing edgeline along which a dimension of the depression in a width directionperpendicular to an axis of the piston pin hole becomes smaller from acenter in a direction along the axis of the piston pin hole toward apiston outer periphery in the direction along the axis of the piston pinhole, and a constant edge line along which the dimension of thedepression in the width direction is maintained to be generally constanttoward the piston outer periphery in the direction along the axis of thepiston pin hole. The changing edge line and the constant edge line arecontinuously formed with an inflection part provided therebetween.

According to these configurations, a center tumble flow is generated onthe center side with respect to the “inflection part,” while a sidetumble flow is generated on the outer side with respect to the“inflection part.” The functions of the center tumble flow and the sidetumble flow are the same as those in the aspects described above. Thus,also according to the third aspect, it is possible to utilize most ofthe fluid energy given to intake air to generate a tumble flow whichflows in the direction perpendicular to the tumble flow axis (axis ofthe piston pin hole), and to promote evaporation and atomization of fuelin the entire cylinder.

Still further, a fourth aspect of the present invention provides apiston for an internal combustion engine including: a top surface havinga valve recess for avoiding interference with a valve and a depression.A region where the valve recess is formed and a region where thedepression is formed are adjacent to each other. An edge line definedbetween the region where the valve recess is formed and the region wherethe depression is formed includes a changing edge line along which adimension of the depression in a width direction perpendicular to atumble flow axis becomes smaller from a center in a direction along thetumble flow axis toward a piston outer periphery in the direction alongthe tumble flow axis, a constant edge line along which the dimension ofthe depression in the width direction is maintained to be generallyconstant toward the piston outer periphery in the direction along thetumble flow axis, and an inflection part connecting the changing edgeline and the constant edge line.

The piston may further include an outer peripheral part having a pistonpin hole through which a piston pin is to be inserted, and an edge linedefined between the region where the valve recess is formed and theregion where the depression is formed includes a changing edge linealong which a dimension of the depression in a width directionperpendicular to an axis of the piston pin hole becomes smaller from acenter in a direction along the axis of the piston pin hole toward apiston outer periphery in the direction along the axis of the piston pinhole, a constant edge line along which the dimension of the depressionin the width direction is maintained to be generally constant toward thepiston outer periphery in the direction along the axis of the piston pinhole, and an inflection part connecting the changing edge line and theconstant edge line.

Also according to these configurations, a center tumble flow isgenerated in a part of the depression defined by the “changing edgeline” on the center side with respect to the “inflection part,” while aside tumble flow is generated in a part of the depression defined by the“constant edge line” on the outer side with respect to the “inflectionpart.” The functions of the center tumble flow and the side tumble floware the same as those in the aspects described above. Thus, alsoaccording to this aspect, it is possible to utilize most of the fluidenergy given to intake air to generate a tumble flow which flows in thedirection perpendicular to the tumble flow axis, and to promoteevaporation and atomization of fuel in the entire cylinder.

The fourth aspect is particularly effective in causing the center tumbleflow and the side tumble flow to fulfill their functions in the pistonin which the region where the valve recess is formed and the regionwhere the depression is formed adjacent to (or overlap) each other.

The part of the depression where the dimension in the width direction islargest may be set as follows. Provided that the valve recess includestwo intake-side valve recesses provided adjacent to each other and twoexhaust-side valve recesses provided adjacent to each other, thedimension of the depression in the width direction perpendicular to thetumble flow axis is set to be largest in a region between anintermediate position between the intake-side valve recesses and anintermediate position between the exhaust-side valve recesses.

Likewise, the dimension of the depression in the width directionperpendicular to the axis of the piston pin hole is set to be largest ina region between an intermediate position between the intake-side valverecesses and an intermediate position between the exhaust-side valverecesses.

According to these configurations, it is possible to generate a tumbleflow with the largest outside diameter at the center of the piston topsurface, allowing a tumble flow of an ideal shape optimum forevaporation and atomization of fuel to be formed in the cylinder.

A fifth aspect of the present invention provides an internal combustionengine including: a cylinder having a cylinder bore; and the pistondescribed above disposed in the cylinder bore. That is, any one of thepistons according to the first to fourth aspects described above isdisposed in the cylinder bore. The piston reciprocates in the cylinderbore to generate power as an air-fuel mixture is combusted in acombustion chamber.

A sixth aspect of the present invention provides a piston for aninternal combustion engine including a top surface having a depression.The depression includes a center region formed at a center in alongitudinal direction of the depression and a side region continuouslyformed on both outer sides of the center region in the longitudinaldirection. The center region has a reduction part in which a dimensionof the center region in a width direction perpendicular to thelongitudinal direction is gradually reduced toward a piston outerperiphery in the longitudinal direction. The side region has a constantpart in which a dimension of the side region in the width direction issubstantially uniform in the longitudinal direction.

According to the present invention, it is possible to utilize most ofthe fluid energy given to intake air to generate a tumble flow whichflows in the direction perpendicular to the tumble flow axis (axis ofthe piston pin hole), to generate a tumble flow generally in the entirecylinder, and to promote evaporation and atomization of fuel in theentire cylinder. As a result, the combustion efficiency of fuel in thecombustion chamber can be increased, and the exhaust emission and thefuel consumption rate can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description of exampleembodiments with reference to the accompanying drawings, wherein likenumerals are used to represent like elements and wherein:

FIG. 1 is a sectional view showing the construction of a combustionchamber of an engine and its surrounding components in accordance withan embodiment of the present invention, showing the state where a tumblecontrol valve is fully closed;

FIG. 2 shows the schematic construction of an intake system in the statewhere the tumble control valve is fully open;

FIG. 3 is a perspective view of a piston;

FIG. 4 is a plan view of the piston;

FIG. 5 is a sectional view taken along the line V-V in FIG. 4;

FIG. 6 is a sectional view taken along the line VI-VI in FIG. 4;

FIG. 7 is a sectional view taken along the line VII-VII in FIG. 4;

FIG. 8 is a sectional view taken along the line VIII-VIII in FIG. 4;

FIG. 9 is a sectional view of the piston at the position correspondingto the line IX-IX in FIG. 5;

FIG. 10A shows the results of an experiment conducted to confirm theretardation limit;

FIG. 10B shows the results of an experiment conducted to confirm thelean limit of the air-fuel ratio;

FIGS. 11A and 11B are each a plan view of a piston top surface inaccordance with a modification of a tumble flow maintaining depression;

FIG. 12 is a plan view of a piston top surface having a depression of atypical conventional shape; and FIG. 13 is a plan view of a piston topsurface having a small depression.

DETAILED DESCRIPTION OF EMBODIMENTS

A description will hereinafter be made of an embodiment of the presentinvention with reference to the drawings. In this embodiment, a pistonfor use in multi-cylinder (for example, inline four-cylinder) gasolineengines for automobiles is described.

FIG. 1 is a sectional view showing the construction of a combustionchamber 10 of an engine (internal combustion engine) 1 and itssurrounding components in accordance with this embodiment. As shown inFIG. 1, the engine 1 in accordance with this embodiment is a four-valvemulti-cylinder gasoline engine including two intake valves 12 and twoexhaust valves 22 for each cylinder. In FIG. 1, only one cylinder isshown, and only one intake valve 12 and one exhaust valve 22 are shown.

A piston 2 which can reciprocate vertically is provided in each cylinder1 a of the engine 1. The piston 2 is coupled to a crankshaft (not shown)via a connecting rod 8 so that reciprocating movement of the piston 2 isconverted into rotation of the crankshaft by the connecting rod 8.

A cylinder head 1 b is attached to the upper part of the cylinder 1 a.The combustion chamber 10 is defined by the cylinder head 1 b, thecylinder 1 a (cylinder bore inner wall), and the piston 2. An ignitionplug 3 is disposed at the upper part of the combustion chamber 10.

An intake port 11 and an exhaust port 21 formed in the cylinder head 1 bare respectively connected to the combustion chamber 10 of the engine 1.The intake valve 12 is provided between the intake port 11 and thecombustion chamber 10. Opening and closing the intake valve 12 allowsand blocks communication between the intake port 11 and the combustionchamber 10. Also, the exhaust valve 22 is provided between the exhaustport 21 and the combustion chamber 10. Opening and closing the exhaustvalve 22 allows and blocks communication between the exhaust port 21 andthe combustion chamber 10. The intake valve 12 and the exhaust valve 22are respectively opened and closed by rotation of an intake camshaft andan exhaust camshaft to which rotation of the crankshaft is transmitted.

An intake passage 13 a formed in an intake manifold 13 is connected tothe intake port 11. Also, an exhaust passage formed in an exhaustmanifold (not shown) is connected to the exhaust port 21.

An air cleaner (with an airflow meter) 5 is provided upstream of theintake passage 13 a. In addition, an electronically controlled throttlevalve 4 for adjusting the intake air amount of the engine 1 and so forthare disposed in the intake passage 13 a.

An injector 6 for fuel injection is disposed at the intake port 11. Theinjector 6 is supplied with fuel at a predetermined pressure from a fueltank (not shown) by a fuel pump, and injects the fuel into the intakeport 11. The injected fuel is mixed with intake air to form an air-fuelmixture, which is inducted into the combustion chamber 10 of the engine1. The air-fuel mixture inducted into the combustion chamber 10 iscompressed in the compression stroke of the engine 1, and then isignited by the ignition plug 3 to burn (expansion stroke). Thecombustion of the air-fuel mixture in the combustion chamber 10reciprocates the piston 2, which rotates the crankshaft as an outputshaft.

The operation state of the engine 1 described above is controlled by anECU (electronic control unit) 7. The ECU 7 includes a CPU, a ROM, a RAM,a backup RAM, and so forth (not shown). The ROM stores various controlprograms, maps to be referenced when executing these control programs,and other data. The CPU executes various operations based on the controlprograms and the maps stored in the ROM. The RAM is a memory fortemporarily storing the results of the operations in the CPU and datainput from respective sensors. The backup RAM is a nonvolatile memoryfor storing data to be saved, for example when the engine 1 is stopped.

The ECU 7 performs various control of the engine 1 by controllingvarious components such as the injector 6, an igniter of the ignitionplug 3, and a throttle motor of the throttle valve 4 based on outputsfrom various sensors (not shown) such as a water temperature sensor, anairflow meter, an intake air temperature sensor, a throttle positionsensor, a crank position sensor (engine speed sensor), a cam positionsensor, and an accelerator position sensor. The ECU 7 further controls atumble control valve 15 to be described later.

A description is now made of the intake system in which a tumble flow inthe cylinder is adjusted by controlling the tumble control valve 15.

As shown in FIG. 1, a partition wall 14 having a predetermined lengthand extending along the longitudinal direction of the intake port 11(the direction of an intake air flow) is provided in the intake port 11.The partition wall 14 and the tumble control valve 15 constitute atumble control mechanism for controlling a tumble flow. The partitionwall 14 is disposed so as not to interfere with fuel spray F injectedfrom the injector 6.

The partition wall 14 divides a part of the space inside the intake port11 into two, upper and lower portions to form an upper flow path 11 aand a lower flow path 11 b at the entrance of the intake port 11. Thetumble control valve 15 is provided upstream of the two, upper and lowerflow paths 11 a, 11 b. The tumble control valve 15 is disposed in theintake manifold 13 connected upstream of the intake port 11.

The tumble control valve 15 includes a plate-like valve element 15 a anda rotary shaft 15 b supporting an end of the valve element 15 a. Anactuator 15 c such as a motor is coupled to the rotary shaft 15 b. Whenthe actuator 15 c is driven, the opening of the valve element 15 a, thatis, the opening of the tumble control valve 15, is adjusted.

The opening of the tumble control valve 15 is controlled by the ECU 7.In the operation state where a tumble flow is required (when the engine1 is cold or in the low to middle speed range, for example), the valveelement 15 a is controlled to the position shown in FIG. 1 (half-openposition). When the engine 1 has been warmed up or is in the high speedrange, for example, the valve element 15 a is controlled to the positionshown in FIG. 2 (full-open position).

The operation of the engine 1 including such an intake system is brieflydescribed below.

First, when the intake valve 12 opens and the piston 2 moves downward inthe intake stroke of the engine 1, intake air passes through the gaparound the intake valve 12 to flow into the combustion chamber 10. Atthis time, in the case where the engine 1 is in the high speed range orhas been warmed up, for example, the valve element 15 a of the tumblecontrol valve 15 is controlled to the full-open position shown in FIG.2. In the state where the tumble control valve 15 is in the full-openposition, intake air flows into both the upper flow path 11 a and thelower flow path 11 b to flow through the gap around the intake valve 12substantially uniformly. Thus, a relatively week airflow is generated inthe combustion chamber 10 to generate only a slight tumble flow.

In contrast, in the case where the engine 1 is in the low to middlespeed range or cold, for example, the valve element 15 a of the tumblecontrol valve 15 is controlled to the half-open position shown in FIG.1, so that most of the intake air passes through the upper flow path 11a to flow into the combustion chamber 10. Thus, a strong tumble flow isgenerated in the combustion chamber 10. Such a tumble flow generatedagitates in the cylinder to promote evaporation and atomization of fuel,achieving excellent fuel property of fuel in the combustion chamber 10in the expansion stroke.

A description is now made of the construction of the piston 2. Thepiston 2 in accordance with this embodiment is formed by casting analuminum alloy. As shown in FIG. 1, the small end of the connecting rod8 is coupled to a piston pin 2 d. A piston pin hole 2 e through whichthe piston pin 2 d is inserted is formed in the outer peripheral part ofthe piston 2. The axis of the piston pin hole 2 e is substantiallyperpendicular to the axis of the piston 2. Piston rings 2 a, 2 b, 2 care respectively mounted in a plurality of (in this embodiment, three)ring grooves formed in the outer peripheral surface of the piston 2.

The shape of the piston top surface is specifically described below.

FIG. 3 is a perspective view of the piston 2 in accordance with thisembodiment. FIG. 4 is a plan view of the piston 2. In the descriptionbelow, for simplicity of description, the direction along the axis L1(see FIG. 4) of the piston pin hole 2 e (see FIG. 3) as the piston 2 isviewed in plan (FIG. 4) is referred to as “X direction,” and thehorizontal direction perpendicular to the axis L1 of the piston pin hole2 e is referred to as “Y direction.” In addition, the direction alongthe axis of the piston 2 (vertical direction) is referred to as “Zdirection.”

As shown in FIGS. 3 and 4, the top surface 23 of the piston 2 inaccordance with this embodiment includes intake-side valve recesses 24,24 formed at positions corresponding to the respective intake valves 12,12, exhaust-side valve recesses 25, 25 formed at positions correspondingto the respective exhaust valves 22, 22, an intake-side squish area 26formed on the outer side of the intake-side valve recesses 24, 24, anexhaust-side squish area 27 formed on the outer side of the exhaust-sidevalve recess 25, 25, and a tumble flow maintaining depression 28 formedto be depressed at the center of the piston top surface 23. That is, thetwo intake-side valve recesses 24, 24 are disposed adjacent to eachother (or aligned along) in the X direction. Also, the two exhaust-sidevalve recesses 25, 25 are disposed adjacent to each other (or alignedalong) in the X direction. The longitudinal direction of the tumble flowmaintaining depression 28 extends in X direction, and coincides with theaxis L1.

The intake-side valve recesses 24, 24 are formed to prevent therespective intake valves 12, 12 from interfering with the piston topsurface 23 when the intake valves 12, 12 are lifted. Since the axis ofeach intake valve 12 is inclined with respect to the axis of the piston2 (Z axis), the valve-facing surface of each intake-side valve recess 24is inclined in the direction generally perpendicular to the axis of theintake valve 12.

Also, the exhaust-side valve recesses 25, 25 are formed to prevent therespective exhaust valves 22, 22 from interfering with the piston topsurface 23 when the exhaust valves 22, 22 are lifted. Since the axis ofeach exhaust valve 22 is inclined with respect to the axis of the piston2 (Z axis), the valve-facing surface of each exhaust-side valve recess25 is inclined in the direction generally perpendicular to the axis ofthe exhaust valve 22.

As described above, the tumble flow maintaining depression 28 is formedin the piston top surface 23. Since the valves do not interfere with theregion where the tumble flow maintaining depression 28 is formed, theintake-side valve recesses 24, 24 and the exhaust-side valve recesses25, 25 are formed on the outer side of and are adjacent to the tumbleflow maintaining depression 28. While the valve-facing surfaces of therespective valve recesses 24, 24, 25, 25 are inclined upward toward thecenter of the piston, the inner surface of the tumble flow maintainingdepression 28 is inclined upward toward the outer side of the piston.Thus, the edge line R of a predetermined shape is defined between theintake-side valve recesses 24, 24 and the tumble flow maintainingdepression 28, and between the exhaust-side valve recesses 25, 25 andthe tumble flow maintaining depression 28. The shape of the edge line Rwill be described later.

The intake-side squish area 26 is formed on the outer side of theintake-side valve recesses 24, 24, and inclined upward (upward in the Zdirection) toward the center of the piston 2. The intake-side squisharea 26 has a function of generating an airflow toward the center of thecombustion chamber 10 (squish flow) by narrowing the space between theintake-side squish area 26 and the cylinder head 1 b in the compressionstroke of the engine 1.

The exhaust-side squish area 27 is formed on the outer side of theexhaust-side valve recesses 25, 25, and inclined upward (upward in the Zdirection) toward the center of the piston 2. As with the intake-sidesquish area 26, the exhaust-side squish area 27 has a function ofgenerating an airflow toward the center of the combustion chamber 10(squish flow) by narrowing the space between the exhaust-side squisharea 27 and the cylinder head 1 b in the compression stroke of theengine 1.

The tumble flow maintaining depression 28 of the piston 2 is formed atthe center of the piston top surface. A detailed description follows.

FIG. 5 is a sectional view taken along the line V-V in FIG. 4. FIG. 6 isa sectional view taken along the line VI-VI in FIG. 4. FIG. 7 is asectional view taken along the line VII-VII in FIG. 4. FIG. 8 is asectional view taken along the line VIII-VIII in FIG. 4. FIG. 9 is asectional view of the piston 2 at the position corresponding to the lineIX-IX in FIG. 5.

As shown in these drawings, the tumble flow maintaining depression 28 isformed to extend across between both vicinities of the outer peripheriesof the piston 2 in the direction along the axis L1 of the piston pinhole 2 e (X direction) through which the piston pin 2 d is inserted, andto extend across between the region where the intake-side valve recesses24, 24 are formed and the region where the exhaust-side valve recesses25, 25 are formed in the direction perpendicular to the axis L1 of thepiston pin hole 2 e (Y direction). The depth at the deepest part of thetumble flow maintaining depression 28 (the depth with respect to theouter periphery of the piston top surface 23; the dimension D in FIG. 5)is set to be about 5% of the outside diameter of the piston 2. Note thatthis figure is not limited thereto.

The tumble flow maintaining depression 28 includes a center tumble flowguiding region (center region) 28A formed at the center in the Xdirection, and side regions 28B, 28B continuously formed on both sidesof the center region 28A (adjacently on both sides in the X direction).In FIG. 4, the center region 28A is hatched with broken lines, while theside regions 28B, 28B are hatched with chain double-dashed lines.

The center region 28A has a reduction part (indicated as Al in FIG. 4)in which the depression width (the dimension in the Y direction) islargest (the dimension T1 in FIG. 4) at the center of the piston topsurface in the X direction, and in which the depression width (thedimension in the Y direction) is gradually reduced toward the pistonouter periphery in the X direction. For example, the depression width atthe part along the VI-VI line in FIG. 4 (the dimension in the Ydirection, or the dimension T2 in FIG. 4; see the sectional view of FIG.6) is smaller than the width at the center (T1). As described above, thedepression width is gradually reduced toward the piston outer peripheryin the X direction in the reduction part A1. The edge line R in thereduction part A1 can be considered as the “changing edge line” of thepresent invention. Specifically, the dimension T1 is set to about 60% ofthe outside diameter of the piston 2, and the dimension T2 is set toabout 55% of the outside diameter of the piston 2. Also, the length ofthe reduction part A1 is set to about 45% of the outside diameter of thepiston 2. Note that these figures are not limited thereto. In FIG. 4,the range (an intermediate position) between the two intake-side valverecesses is shown by the dotted line P1, while the range (anintermediate position) between the two exhaust-side valve recesses isshown by the dotted line P2. The dimension of the depression in the Ydirection is largest in the region defined by the dotted lines P1 and P2and the lines connecting the respective opposing ends of P1 and P2 witheach other, that is, the region between P1 and P2 opposing each other.

The intake-side squish area 26 and the exhaust-side squish area 27preferably have as large an area as possible. On the other hand, thedepression width (the dimension in the Y direction) is set to be largeat the center in the X direction, as described above. Thus, in theregion between the intake-side valve recesses 24, 24, the inclinationangle of the inclined surface S1 extending from the outer periphery ofthe center region 28A to the intake-side squish area 26 is set to belarger (for example, 50° with respect to the horizontal direction) thanthat of a typical conventional piston. Likewise, in the region betweenthe exhaust-side valve recesses 25, 25, the inclination angle of theinclined surface S2 extending from the outer periphery of the centerregion 28A to the exhaust-side squish area 27 is set to be larger (forexample, 50° with respect to the horizontal direction) than that of atypical conventional piston.

The edge line R in the reduction part A1 is arcuate with a constantcurvature. The edge line R is not limited thereto, and may be curvedwith a gradually changing curvature or generally straight.

On the other hand, the side regions 28B, 28B have a constant part(indicated as A2 in FIG. 4) of which outer edges extend generally inparallel to the X direction. Thus, the depression width at the partalong the VII-VII line in FIG. 4 (the dimension in the Y direction, orthe dimension T3 in FIG. 4; see the sectional view of FIG. 7) isgenerally equal to the depression width at the part along the VIII-VIIIline in FIG. 4 (the dimension in the Y direction, or the dimension T4 inFIG. 4; see the sectional view of FIG. 8), for example. As describedabove, the depression width in the Y direction is generally uniform inthe constant part A2. The edge line R in the constant part A2 can beconsidered as the “constant edge line” of the present invention.Specifically, the dimensions T3 and T4 are set to about 50% of theoutside diameter of the piston 2. Also, the length of each constant partA2 is set to about 13% of the outside diameter of the piston 2. Notethat these figures are not limited thereto.

An inflection part R1 is provided at the boundary between the reductionpart A1 and the constant parts A2, A2, where the curvature of the edgeline R changes between that of the reduction part A1 and that of theconstant parts A2, A2. The inflection part R1 smoothly connects the edgeline R of the reduction part A2 and the edge line R of the constant partA2. Thus, the part of the tumble flow maintaining depression 28 on thecenter side with respect to the inflection part R1 is defined as thecenter region 28A, while the part of the tumble flow maintainingdepression 28 on the outer side (in the X direction) with respect to theinflection part RI is defined as the side regions 28B, 28B. Theinflection part R1 exists on each edge line between the intake-sidevalve recesses 24, 24 and the tumble flow maintaining depression 28, andon each edge line between the exhaust-side valve recesses 25, 25 and thetumble flow maintaining depression 28. That is, the inflection part R1exists at four positions on the piston top surface 23.

The symbol C in FIG. 3 indicates the outer peripheral shape of the partto be pushed in casting the piston 2, the inner region of which (whichoccupies a most part of the center region 28A) is generally flat. Therobustness of the piston 2 is increased by positioning the part to bepushed in the center region 28A which is relatively large.

A description is now made of how a tumble flow is generated. Since thetumble flow maintaining depression 28 includes the center region 28A andthe side regions 28B, 28B as described above, intake air flowing intothe cylinder when the engine 1 is driven generates a tumble flowincluding a center tumble flow flowing along the center region 28A and aside tumble flow flowing along the side regions 28B, 28B.

At the center of the center region 28A in the X dimension, thedepression width (the dimension in the Y direction) is secured to berelatively large, so that a center tumble flow having a large diameterand a flow line extending in the Y direction is generated (see thetumble flow shown in FIG. 5). In addition, in the reduction part A1 ofthe center region 28A, the depression width (the dimension in the Ydirection) is gradually reduced toward the outer side in the Xdirection, so that the diameter of the tumble flow becomes graduallysmaller toward the piston outer periphery. That is, the diameter of thetumble flow becomes gradually smaller as the in-cylinder length in the Ydirection becomes gradually smaller toward the piston outer periphery soas to generate an ideal center tumble flow (that is not influenced bythe cylinder bore inner wall) with its flow line extending in the Ydirection and not disturbed (see the tumble flow shown in FIG. 6).

On the other hand, since the side regions 28B, 28B have the constantparts A2, A2 extending in the X direction, the diameter of the sidetumble flow which flows in the side regions 28B, 28B does not becomesmaller toward the piston outer periphery, but is generally uniform overthe entire side regions 28B, 28B. That is, a side tumble flow with agenerally uniform outside diameter smaller than that of the centertumble flow is generated in the side regions 28B, 28B (see the tumbleflow shown in FIGS. 7 and 8). Thus, the side tumble flow is hardlyinfluenced by the cylinder bore inner wall adjacent in the X direction,and has a flow line extending in the Y direction. In addition, sincethere is almost no influence of the cylinder bore inner wall asdescribed above, a tumble flow is generated in the vicinity of the outerperiphery of the piston top surface 23, which excellently agitates inboth outer sides of the cylinder in the axial direction of the tumbleflow (X direction) to consequently promote evaporation and atomizationof fuel in the entire cylinder.

As described above, according to the piston 2 in accordance with thisembodiment, it is possible to utilize most of the fluid energy given tointake air to generate a tumble flow which flows along the Y direction,and to promote evaporation and atomization of fuel in the entirecylinder. As a result, the combustion efficiency of fuel in thecombustion chamber 10 can be increased, and the exhaust emission and thefuel consumption rate can be improved.

In addition, since the total capacity of the tumble flow maintainingdepression 28 is restricted by the reduction part A1 in the centerregion 28A narrowing the width of the tumble flow maintaining depression28, the compression ratio of the combustion chamber 10 can be increased.Thus, it is possible to easily realize the engine 1 with a compressionratio exceeding 10, for example, while maintaining the effect describedabove (to increase the combustion efficiency to improve the exhaustemission and the fuel consumption rate).

A description is now made of an experiment conducted to confirm theeffect of the embodiment described above and the results of theexperiment. In this experiment, a piston with a top surface having adepression of a conventional shape (such as shown in FIG. 12) and thepiston 2 in accordance with the above embodiment are each assembled intoan engine (inline four-cylinder gasoline engine) to compare theretardation limit (the limit retardation amount at which the enginestalls) by gradually retarding the ignition timing of the ignition plug3 (Experiment 1). Also, the lean limit (the limit air-fuel ratio atwhich the engine stalls) is compared by making the air-fuel ratiogradually leaner (Experiment 2).

The results of Experiment 1 are shown in FIG. 10A, and the results ofExperiment 2 are shown in FIG. 10B.

As shown in FIG. 10A, the piston 2 with the top surface 23 having thedepression 28 of the shape in accordance with the present inventionachieved a retardation limit of approximately 12% higher than thatachieved by the piston with a top surface having a depression of aconventional shape. That is, it was confirmed that in the case where thepiston 2 in accordance with the present invention was used, the ignitiontiming could be retarded significantly more than the case where theconventional piston was used without stalling the engine, so that acatalytic converter could be activated (heated) quickly by increasingthe ignition retardation amount in particular when starting the engine 1while it is cold.

Also, as shown in FIG. 10B, the piston 2 with the top surface 23 havingthe depression 28 of the shape in accordance with the present inventionachieved a lean limit of the air-fuel ratio of approximately 30% higherthan that achieved by the piston with a top surface having a depressionof a conventional shape. That is, it was confirmed that in the casewhere the piston 2 in accordance with the present invention was used,the air-fuel ratio could be made significantly leaner than the casewhere the conventional piston was used without stalling the engine, sothat the fuel consumption rate was significantly improved.

A description is now made of modifications of the tumble flowmaintaining depression 28. FIGS. 11A and 11B are each a plan view of thepiston top surface 23 showing a modification of the tumble flowmaintaining depression 28 (in which valve recesses are not shown).

In the modification shown in FIG. 11A, the edge line R in the centerregion 28A is changed such that the curvature radius increases towardthe piston outer periphery in the X direction, and the depression widthin the Y direction in the side regions 28B, 28B is set to be smallerthan that in the above embodiment.

Also, in the modification shown in FIG. 11B, the extending direction ofthe edge line R, which defines the outer periphery of the tumble flowmaintaining depression 28, changes by approximately 90° in theinflection part R1 at the boundary between the reduction part A1 and theconstant parts A2, A2.

In the embodiment and the modifications described above, the presentinvention is applied to a piston for use in multi-cylinder gasolineengines for automobiles. The present invention is not limited thereto,and may be applied to diesel engines for automobiles. The presentinvention may also be applied to engines for other than automobiles. Thespecification of the engine, such as the number of cylinders and whetheror not the engine has a tumble control valve 15, is not specificallylimited.

1. A piston for an internal combustion engine, comprising: a top surfacehaving a valve recess for avoiding interference with a valve and atumble flow maintaining depression, wherein a region where the valverecess is formed and a region where the depression is formed areadjacent to each other, and an edge line defined between the regionwhere the valve recess is formed and the region where the depression isformed includes a changing edge line along which a dimension of thedepression in a width direction perpendicular to a tumble flow axisbecomes smaller from a center in a direction along the tumble flow axistoward a piston outer periphery in the direction along the tumble flowaxis, a constant edge line along which the dimension of the depressionin the width direction is maintained to be generally constant toward thepiston outer periphery in the direction along the tumble flow axis, andan inflection part connecting the changing edge line and the constantedge line.
 2. The piston according to claim 1, wherein the valve recesscomprises two intake-side valve recesses provided adjacent to each otherand two exhaust-side valve recesses provided adjacent to each other; andthe dimension of the depression in the width direction is set to belargest in a region between an intermediate position between theintake-side valve recesses and an intermediate position between theexhaust-side valve recesses.
 3. The piston according to claim 1, furthercomprising: an outer peripheral part having a piston pin hole throughwhich a piston pin is to be inserted, wherein an axis of the piston pinhole is substantially parallel to the tumble flow axis.
 4. The pistonaccording to claim 3, wherein the dimension of the depression in thewidth direction is largest substantially at the center of the topsurface in the direction along the axis of the piston pin hole.
 5. Thepiston according to claim 1, wherein the dimension of the depression inthe width direction is largest substantially at the center of the topsurface in the direction along the tumble flow axis.
 6. An internalcombustion engine comprising: a cylinder having a cylinder bore; and thepiston according to claim 1 disposed in the cylinder bore, wherein thepiston reciprocates in the cylinder bore to generate power as anair-fuel mixture is combusted in a combustion chamber.
 7. A piston foran internal combustion engine, comprising: a top surface having a valverecess for avoiding interference with a valve and a tumble flowmaintaining depression; and an outer peripheral part having a piston pinhole through which a piston pin is to be inserted, wherein a regionwhere the valve recess is formed and a region where the depression isformed are adjacent to each other, and an edge line defined between theregion where the valve recess is formed and the region where thedepression is formed includes a changing edge line along which adimension of the depression in a width direction perpendicular to anaxis of the piston pin hole becomes smaller from a center in a directionalong the axis of the piston pin hole toward a piston outer periphery inthe direction along the axis of the piston pin hole, a constant edgeline along which the dimension of the depression in the width directionis maintained to be generally constant toward the piston outer peripheryin the direction along the axis of the piston pin hole, and aninflection part connecting the changing edge line and the constant edgeline.
 8. The piston according to claim 7, wherein the valve recessincludes two intake-side valve recesses provided adjacent to each otherand two exhaust-side valve recesses provided adjacent to each other; andthe dimension of the depression in the width direction is set to belargest in a region between an intermediate position between theintake-side valve recesses and an intermediate position between theexhaust-side valve recesses.
 9. The piston according to claim 7, whereinthe dimension of the depression in the width direction is largestsubstantially at the center of the top surface in the direction alongthe axis of the piston pin hole.
 10. An internal combustion enginecomprising; a cylinder having a cylinder bore; and the piston accordingto claim 7 disposed in the cylinder bore, wherein the pistonreciprocates in the cylinder bore to generate power as an air-fuelmixture is combusted in a combustion chamber.
 11. A piston for aninternal combustion engine, comprising a top surface having a tumbleflow maintaining depression, wherein the depression includes a centerregion formed at a center in a longitudinal direction of the depressionand a side region continuously formed on both outer sides of the centerregion in the longitudinal direction, the center region has a reductionpart in which a dimension of the center region in a width directionperpendicular to the longitudinal direction is gradually reduced towarda piston outer periphery in the longitudinal direction, the side regionhas a constant part in which a dimension of the side region in the widthdirection is substantially uniform in the longitudinal direction, andwherein the top surface further includes a valve recess adjacent to thedepression, and an edge line defined between the valve recess and thedepression includes a changing edge line along which a dimension of thedepression in the width direction becomes smaller from a center in thelongitudinal direction toward the piston outer periphery in thelongitudinal direction, a constant edge line along which the dimensionof the depression in the width direction is maintained to be generallyconstant toward the piston outer periphery in the longitudinaldirection, and an inflection part connecting the changing edge line andthe constant edge line.
 12. The piston according to claim 11, whereinthe valve recess includes two intake-side valve recesses providedadjacent to each other and two exhaust-side valve recesses providedadjacent to each other, the height between the bottom of the depressionand the edge line is shorter than the height between the bottom of thedepression and a ridge that is formed between the depression and asquish area formed on the outer side of the depression, and is formedbetween the two intake-side recesses in the longitudinal direction. 13.The piston according to claim 11, wherein the valve recess includes twointake-side valve recesses provided adjacent to each other and twoexhaust-side valve recesses provided adjacent to each other, the heightbetween the bottom of the depression and the edge line is shorter thanthe height between the bottom of the depression, and a ridge that isformed between the depression and a squish area formed on the outer sideof the depression, and is formed between the two exhaust-side recessesin the longitudinal direction.
 14. The piston according to claim 13,wherein the longitudinal direction of the depression substantiallycorresponds to a tumble flow axis.
 15. The piston according to claim 13,further comprising an outer peripheral part having a piston pin holethrough which a piston pin is to be inserted, wherein the longitudinaldirection of the depression substantially corresponds to an axis of thepiston pin hole.